The present invention relates to a method for detailed mapping of the thickness and topography of buoyant objects, especially ships"" hulls, by means of a beam carrying an array of ultrasound sensors, together with a device for implementation of the method.
The term ultrasound sensor should be understood here to refer to a measuring device comprising means both for transmission of acoustic signals towards or into a structure, and means for measuring/recording acoustic signals reflected from the structure, possibly also means for processing the result of the measurement.
The invention is intended amongst other things for use in connection with inspection of a ship""s bottom, as an objective evaluation of the state of the ship""s hull with regard to corrosion. This is important in the context of classification approval, where there is a requirement for the surveying of corrosion with subsequent repairs according to certain criteria. By performing inspections regularly, or in good time before classification approval/repair, the shipowner obtains a sound decision basis for planning and selecting the most rational maintenance/repair measures.
Contact with a corrosive environment causes ships"" bottoms to be exposed to internal corrosion. This applies particularly to tankers carrying crude oil, where a corrosive environment is created in the bottom of the tanks on account of the acid content of the crude oil, and water. For the main classification every 5 years, therefore, it is obligatory to inspect the ship for corrosion. During the inspection the thickness of the steel plates is measured both in order to check the strength of the hull and to prevent leaks due to pit corrosion. If the thickness falls short of the requirements stipulated by the classification society, the whole or parts of the steel plates concerned must be replaced.
The current traditional measuring method is based on manual spot measurements undertaken from the inside or occasionally by means of divers from the outside when the ship is in port. This method is not particularly suitable for internal inspection of oil tanks, since the tank has to be cleaned and ventilated before inspection, and this is a time-consuming procedure. Moreover, it can be difficult for the operator to gain access. In addition to this, more and more ships are equipped With a double bottom, thus making it difficult to inspect the outer hull.
Another problem with manual inspection methods is that they are dependent on the operator for choice of measuring point. There are procedures for preventing serious errors of choice, but full covering with an automatic measuring system in a fixed grid will always be better.
Another measuring method is described in NO-B-179,926, where a small sensor array is employed mounted on a self-propelled vessel which scans the ship""s bottom. The vessel is equipped with wheels, which provide a constant distance between the vessel and the ship""s bottom. The position of the sensor array is measured continuously by means of a hydroacoustic positioning system, thus permitting the measurements to be assembled later to form a complete map of the ship""s bottom. This method is based on movement and positioning of a small vessel which rolls on self-propelled wheels under the ship""s hull, a procedure which requires complicated positioning equipment as well as being time and resource-demanding (an extra 24 hours in port for a vessel represents a substantial expenditure). Furthermore, powerful flow forces at the locations where the survey is conducted will also complicate the operation. Since the vessel is small, it is not possible to measure topography (dents, etc.) by this method. The method is specially developed for use in large oil tankers, and in addition it does not permit inspection of the curved transition between ship""s bottom and ship""s side (the apparent horizon).
U.S. Pat. No. 3,910,104 describes an apparatus and a method for ultrasonic testing of metal plates. The apparatus comprises a container filled with water and with an opening on the top, the object which has to be inspected being in contact with the water through the opening. Under the object and inside the container there is placed a beam equipped with an array of ultrasound sensors. The measuring is conducted by passing the object over the apparatus while the beam is moved backwards and forwards. The apparatus is intended for single plates and is not at all suitable for inspection of ships"" hulls or large buoyant objects.
None of the said methods permit inspection of large objects such as, e.g., ships"" hulls in a rapid and cost-effective manner. In particular the known methods fail to provide a fast and efficient mapping of the objects"" topography and thickness.
The object of the invention is to solve the above-mentioned problems in the prior art by means of a method and a device which permit precise and rapid inspection of buoyant objects. This is achieved according to the invention by means of a beam carrying an array of ultrasound sensors, and by the object and the beam, which can be raised and lowered, and which extends along the whole or a substantial part of the width of the object, being moved relatively to each other in the object""s longitudinal direction while the ultrasound sensors inspect the whole or a substantial part of the width of the object, thus permitting complete mapping to be achieved by means of one or a small number of passes relatively between the object and the beam.
The method and the device according to the invention are characterized by that which is set forth in the attached patent claims.
According to the prior art plate thickness and distance from the sensor can be calculated on the basis of each individual ultrasound measurement when the velocity of sound in the plate material and the water are known. Since the beam in a preferred embodiment assumes a known geometric shape (preferably a rectilinear shape), the shape of the object in a sectional plane through the beam can be mapped. In this manner the object""s thickness and topography can be completely mapped by means of one or a small number of passes relatively between the object and the beam.
If the ultrasound sensors are placed in an articulated beam it may be appropriate to place an extra set of distance measuring devices in a straight line in order to measure distance/topography. Alternatively, each joint can have angle gauges for determining topography.
In a preferred embodiment of the method, the beam is stationary while the object is moved past the beam. In an embodiment of the device suitable for use in this method, the device comprises, means for determining the beam""s position and prevent the beam""s movement in the object""s longitudinal direction, together with an adjustable securing device for restricting the beam""s movement in its longitudinal direction (corresponding to the object""s transverse direction). Thus the invention entails a complete mapping of the thickness of the whole or a substantial part of the object, the latter being drawn over a large array of ultrasound sensors. The sensors may be stationary or they may be arranged on a body which can be moved from one side of the beam to the other. When employed for determination of the thickness and topography of ships"" hulls, the ship is moved across the beam under its own power, pulled by tugs, or by a winch while ultrasound sensors measure the plate thickness. While the ship is passing the beam, the measurements are assembled to form a detailed map of the thickness of the ship""s flat bottom. The position of the measuring points in the latitudinal direction is given by the geometry of the equipment, while the position in the longitudinal direction is determined by, e.g. measuring wheels or other suitable positioning systems. The distance between the ship and the beam (depth setting) is adjusted by means of lifting means secured by floating bodies and/or securing means located on land. The device may be placed beside or in a lock, dock or at another practical location often passed by ships.
In a second embodiment of the invention, which is adapted to inspection of ships on the open sea, the floating body containing the beam consists of service boats equipped with position sensors for determining and controlling the relative movement between the beam and the ship.
In a further embodiment of the invention, which is adapted to use in connection with loading and unloading operations on a ship, the lifting means are secured by a service boat on the seaward side of the ship and a wagon on the quay side of the ship.
According to the invention the distance between the object and the beam is adjusted by moving the beam until it is located directly below the object.
As distance elements, fixed or rotating brushes, or high-pressure hose nozzles may also be employed, with the result that cleaning and inspection are performed in one operation while achieving optimal inspection distance in a simple manner.
The resolution along the beam will be given by the distance between the ultrasound sensors, e.g. 5 cm, while the resolution along the other axis (in the beam""s transverse direction) is given by the ratio between the relative velocity and how often the measurements are logged. With modern distributed measuring and storing it is possible to attain a resolution of 5 cm along both axes at a relative velocity of, e.g., 0.25 m/s. The high resolution and the accurate positioning of the sensors on the beam permit control of a large number of measuring points with great accuracy. The measurements can subsequently be compared in order to establish the object""s state of corrosion.
The invention also permits scanning of the transition between ship""s bottom and ship""s side together with the ship""s side. For the part which is located above water a device may be employed which permits water contact between the sensor array and the ship""s hull, each sensor, e.g., being provided with a jet of water which ensures the presence of a communication channel for ultrasound. A specially preferred embodiment of the invention for use in this connection comprises an xe2x80x9carticulatedxe2x80x9d beam, i.e. a beam consisting of at least two parts interconnected by an articulated element, and where the parts can be pivoted in order to follow the contour of the object. The joint(s) may be of a type which offers limited angular alteration. For further adjustment of the distance between the beam and the ship""s bottom/side, the beam may be provided with floating bodies and/or distance elements on or near the joint(s). The distance elements are also useful when employing a xe2x80x9crigidxe2x80x9d (non-articulated) beam.
In the case of both a rigid and articulated beam it is important for the beam to be stable with regard to wave and flow forces, while at the same time keeping down the beam""s weight. This is solved in a preferred embodiment by the beam consisting of a trusswork and being made of, e.g., steel, aluminium, titanium, or a fibre-reinforced composite material.
Due to the fact that the beam constitutes a fixed reference point along the whole or a substantial part of the object""s width, the system also permits measurement of the topography (dents, etc.) of the ship""s hull. This can be of great interest to insurance companies in connection with settlement of claims, and change of owner.